Acquired immune deficiency syndrome (AIDS) was first
recognized in 1981 as an unprecedented group of opportunistic infections
appearing in individuals with no known predisposition for immune system
dysfunction (Durack, 1981).

An estimated 42 million individuals are currently infected
worldwide, with the vast majority of those in sub-Saharan Africa. Spread of HIV
infection continued at an alarming rate in 2002 with over 5 million new
infections and 3 million deaths (UNAIDS, 2002).

AIDS can be considered a disease of an immune system that is
not able to function effectively. HIV can weaken the immune system to the point
that the infected host is unable to control certain organisms that are usually
controlled by a healthy immune system. On that account, these types of pathogens
are known as "opportunistic" because they are able to express their pathogenic
effect only in association with predisposing factors that weaken the immune
system.

Some of these infections are well-recognized zoonoses that are
naturally transmitted between vertebrates and humans. Others are associated
with, but not directly transmitted by, animals. Aside from these direct
encounters with animals, indirect contact may also occur through such means as
contaminated food and water.

Interactions between animals and humans are truly complex and
health care providers should be aware of the potential role of animals in
infectious diseases of HIV infected patients. This relationship is not limited
to direct contact between humans and animals, but could also be indirect. In the
case of indirect transmission, complicated patterns are usually involved and
knowledge of these is required to understand better the epidemiology of HIV
related infections and to provide medical support to infected
patients.

Treatments able to slow down the rate at which HIV weakens the
immune system are now available as well as effective drugs to treat
opportunistic pathogens. Nevertheless, there is a pressing need to better
clarify the intimate correlation between host and opportunistic pathogens. This
will give insights into the pathogenic pathways and provide scientific
communities with better strategies to prevent, or at least reduce the rate of
opportunistic infections which occur in HIV infected patients.

The aim of this guideline is to outline the most important
zoonoses that play a significant role in the epidemiology of AIDS and to provide
a practical and manageable tool for health workers involved in the care of HIV
infected humans.